Buy one, get one free used to be the favoured fast-food deal of the Vunipola brothers at their local KFC. Now they have slimmed down, it is England who are cashing in on the deal.

Billy and Mako Vunipola have revealed that a fast-food diet slowed their careers to a standstill before they were knocked into shape.

England’s fitness gurus were so alarmed at the weight of the Tongan-born brothers that a strict diet and training routine was imposed: cut out the junk food and swap the KFC for protein shakes.

The work has certainly paid off, as Mako will get his first start against Italy at Twickenham and both could be named in a Test 23 for the first time this Sunday.

Brotherly love: The two Vunipola brothers could line up for England on Sunday

ROLLING BACK THE YEARS

If Stuart Lancaster picks the Vunipola brothers and the Youngs brothers to face Italy on Saturday, it will be the first time in 114 years that two sets of brothers have played for England in the same game.

The only time it has happened before was in March 1899, when Scotland beat England 5-0 on the Rectory Field at Blackheath in front of 25,000 supporters.

Pictured are Percy (middle row, second left) and Frank Stout (middle row, far right) of Gloucester and Jos (front row, left) and Jas Davidson (front row, right) of Aspatria (Cumberland), who all featured in that losing side.

The Davidsons and Frank Stout were forwards — forwards didn’t specialise in those simpler times — while Percy Stout was a centre. At full back was Herbert ‘Octopus’ Gamlin, who presumably had only two arms.

Billy, the uncapped Wasps No 8, estimates that he has had to lose the best part of three stones since he joined England’s youth teams at Under 18 level.

The 20-year-old has now trimmed down to a regular 19st 11lb, while prop Mako, 22, claims he is content to be 20st on a ‘good day’, having lost two stones since his youth. Billy puts the change down to Neil Taylor of England’s backroom team, who introduced a new regime when they joined the Under 18 squad.

‘The management sat us down and said we needed to shed a few pounds if we wanted to play to a high level,’ said Billy, before remembering their old ways.

‘We’d starve ourselves by not having breakfast or lunch, then come home and eat so much junk food,’ he said. ‘We’d feast on whatever is quickest, the fastest food we could get our hands on: carbs, bread — our favourite — potato, chocolate.

‘And we’d eat a lot of it. Junk food is so cheap and those “buy one, get one free” deals didn’t help us at all.’ Both believe their excess pounds held them back in England’s age-group teams, having first moved to Britain as youngsters while father Fe’ao, who played as a hooker for Tonga, played and coached here.

Mako recalled visiting his brother in hospital and taking him a meal from a nearby KFC takeaway.

‘I walked straight through the ward and the nurse’s reaction was, “I thought you were supposed to be rugby players”,’ he said.

The delivery of the fried chicken was to aid Billy’s recovery from a virus which had infected discs in his back and also led to a bout of pneumonia. It was all part of a wretched start to the 2011-12 season for Billy. ‘I was in hospital for quite a long time, so to be sat here at England training is quite unbelievable,’ he said.

Big Billy! Vunipola towers above his classmates at the age of 11

Powerful: The strong running prop was a left-field selection by Stuart Lancaster

Putting the work in: Marko Vunipola trains with the England side at training on Tuesday

Billy does seem a little accident prone, including burning himself by forgetting to put the cover on a hot water bottle which he was using to ease his back pain following a game.

‘Usually my mum put a cover over it, but I did not know where it was, so I just threw it over my back. It burnt me and gave me blisters.’

He was also injured when first joining the England senior squad in January, due to a foot problem sustained playing for Wasps.

He was immediately concerned that the more experienced players would think he was only at the Bagshot training ground for the free food.

On form: The Wasps prop has been a constant presence for his club side

Force: Older brother Mako Vunipola has starred for England during the Six Nations

‘It was pretty annoying and awkward but as soon as I started training I was fine.’

The brothers remain in touch with their Tongan roots — father Fe’oa is coaching the Under 20 team.

But Mako said: ‘We are always going to be proud of our ancestry from Tonga, but we feel right at home in England and happily call England home.’